As is known in the glass making art, a melting furnace, made up of an elongated tank topped by a crown, essentially comprises two parts: a refining zone maintained at a high temperature; and, a conditioning zone maintained at a lower temperature, also called a braising zone. In modern furnaces, the refining and conditioning zones are joined by a narrow section called a corset. Raw materials forming the vitrifiable composition are introduced upstream of the refining zone through a charging hole. These raw materials are melted by heating devices such as burners. The molten glass then flows downstream, through the corset, to the conditioning zone, where it is brought to a suitable temperature, to be taken off by drawing for further operations of shaping, drawing, and floating or, more generally, fashioning. To provide the necessary regulation of the glass temperature the conditioning zone may also be provided with cooling devices in addition to the heating devices.
It is not enough, however, merely to bring the glass, which is to be removed for fashioning, to the desired temperature; this glass must also be homogeneous. However, the conditions within a conventional furnace tend to prevent homogeniety. Certain well defined currents are formed in the various regions of the furnace because of differences in temperature and the drawing off the glass. While these currents do create some mixing, they are localized. The most notable of these currents is a forward current usually located immediately in the upper part of the molten glass which flows from the refining zone to the conditioning zone. Meanwhile a return current, located under the forward current, flows in the opposite direction. The combination of these currents, however, is not always sufficient to produce a homogeneous glass. In a given sample of glass, taken at the furnace output, it is possible for the sample to contain masses of glass which have resided in the furnace for periods ranging from several dozen hours to as much as several weeks.
An effort has been made to remedy this drawback by placing in the molten glass bath mechanical or thermal barriers to force the glass currents through passages in the depth of the glass bath, thus causing an increased mixing. It has also been proposed to separate physically two distinct zones of the glass bath by means of floating barriers. These floating barriers are placed along the width of the tank and their function is either to cause a mixing of the bath or to divide the furnace into two successive zones.
It has also been proposed to block only the median part of the molten glass bath by a barrier occupying only a part of the width and height of the bath. This barrier--placed between the refining zone and the conditioning zone and, in particular, in the corset when the furnace includes one--makes it possible to divert the central current of recently formed glass which, in the absence of the barrier, would be the primary source of glass being drawn off from the furnace.